In an automatic transmission for an automobile, the output of an engine is input in a gear type transmission through a torque converter, and in the gear type transmission, the speed is changed by coupling and disconnection of transmission elements such as a clutch and a band brake.
For example, in the case where the speed is changed to the second speed from the first speed, a shift valve is changed over by a speed-changing instruction to increase the band brake pressure above a predetermined value and bring about a torque phase (the state where one pair of friction elements slip but the other pair of friction elements are completely engaged with each other). Then, the band brake pressure is further increased to transfer the torque phase to an inertia phase (the state where both of the two pairs of friction elements slip) causing a change of revolution (see FIG. 6).
At the detection of the timing of the speed change, the transfer of the phase to the inertia phase from the torque phase is detected by the start of falling of the revolution of the engine, and the hydraulic oil pressure is changed and controlled so that an optimum value of hydraulic oil pressure is obtained for each phase.
However, in this conventional apparatus, since the transfer of the phase to the inertia phase from the torque phase is detected by the start of falling of revolution of the engine and the control is performed based on the result of the detection, a problem of a delay of the response of the oil pressure arises, and the oil pressure is not changed immediately at the transfer of the phase to the inertia phase from the torque phase and the controllability is very low.
Under this background, it is a primary object of the present invention to solve the above-mentioned problem and to detect the transient state of the speed change at a proper timing while taking the response delay of the hydraulic oil pressure into consideration.